How to list subpages of active page

Hi guys.

I was wondering how I can list the subpages of an active page in a vertical menu?

Example:

http://www.relevancerecruitment.nl/

Click on ‘Diensten‘ on the left vertical menu.

And you will get what I want…

So, the subpages should not be visible when you click on it, but after the page has loaded…